Electrical coil having integrated capacitance and inductance

ABSTRACT

This coil comprises an annular core surrounded by a plurality of layers of electrically conductive wire windings separated from each other by at least one layer of metal foil, and at least one layer of elastic dielectric material. By selective connection of the wire windings and the intervening foil layers in electric circuits, the coil may be made to function selectively as a choke, filter, signal delay, or as a storage device or absorber for impulse energy. By adding axially extending metal strips to the coil between its layers of windings, the coil is suitable for use in an electric current generator of the magnetomotive variety.

United States Patent Combs 1451 Jan. 25, 1972 [54] ELECTRICAL COIL HAVING 2,592,817 4/1952 McKechnie ..336/6l INTEGRATED CAPACITANCE AND 3,416,111 12/1968 Bogner 336/61 X I T NCE 322,724 7/1885 Jackson et al 336/69 X N A 3,102,245 8/1963 Lawson, Jr 336/205 X [72] Inventor: Van P. Combs, Penfield, NY. 3,391,366 7/1968 Stokkeland et al. 336/234 X 3,394,331 7/1968 Aiken et al. ..336/70 [73] Assgnee' Rmhesm 2,356,229 8/1944 Dunlap ..31o 25 x [22] Filed: Nov. 6, 1970 I Primary Examiner-Thomas .l Kozma [2 H Appl' 87447 AnorneyShlesinger, Fitzsimmons & Shlesinger Related U.S. Appllcatlon Data I ABSTRACT [63] Continuation-impart of Ser. No. 800,383, Feb. 19,

1969, Pat. No. 3,568,040, Thls co1l comprlses an annular core surrounded by a plurahty of layers of electrically conductive wire windings separated [52] U S Cl 336/69 336/84 336/205 from each other by at least one layer of metal foil, and at least A 336/234 one layer of elastic dielectric material. By selective connec- [SI] Int Cl no" 15/14 tion of the wire windings and the intervening foil layers in [58] Field I80 2 electric circuits, the coil may be made to function selectively I234 g, i as a choke, filter, signal delay, or as a storage device or absorber for impulse energy. By adding axially extending metal [56] References Cited strips to the coil between its layers of windings, the coil is suitable for use in an electric current generator of the mag- UNITED STATES PATENTS "elomolive y- 2,008,859 7/1935 Ganz ..336/84 X 6 Claims, 14 Drawing Figures mmwmm 3.638.155

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' VAN P. ooMBs ATTORNEYS PATENTEDJANZSIQIZ 3.838.155 SHEEIQ-UF 4 INVENTOR. VAN P. COM BS m X ATTORNEYS ELECTRICAL COIL HAVING INTEGRATED CAPACITANCE AND INDUCTANCE This invention is a continuation-in-part of my copending US. application Ser. No. 800,383, filed Feb. l9, I969, now US. Pat No. 3,568,030, granted Mar. 2, l97land relates to an electrical coil.

More particularly this application relates to a multiterminal coil having integrated capacitance and inductance elements.

Conventional inductance coils usually are designed in such manner as to eliminate the introduction of any stray capacitance, when the coils are employed in electrical circuits. Typically such a coil comprises insulated wire that is wound on or about a core, which for purposes of developing maximum inductance in the associated coil may be made of soft iron. Generally, opposite ends of the coiled wire form the two terminals by means of which the coil is connected in a circuit.

The insulation for the wire of the coil usually is made from a nonelastic dielectric; and the wire itself is usually made of a good electrical conducting material, such as copper or aluminum. Moreover, when an iron core is used, the core usually is disposed coaxially within the coil.

When a DC current is fed to a conventional coil of the type described, magnetic energy is stored therein, but very little electrostatic energy.

It is an object of this invention to provide a novel electrical coil having integrated inductance and capacitance characteristics which can be incorporated selectively with the coil in an electric circuit.

Another object of this invention is to provide a novel electrical coil having a plurality of layers of winding separated by a plurality of layers of insulated, foil conductors to provide a dynamic electromagnetic energy storage coil.

A further object of this invention is to provide a novel electrical coil which is more versatile than conventional inductance coils.

A still further object of this invention is to provide a multitenninal electrical coil capable selectively of functioning as a high-voltage inductance coil, a filter, delay or oscillator, respectively.

An additional object of this invention is to provide a core for a coil of the type described, which is particularly useful as part of the core of a magnetomotive electric current generator.

Other objects of the invention will be apparent hereinafter from the specification and from the recital of the appended claims, particularly when read in conjunction with the accom panying drawings.

In the drawings:

FIG. I is a side elevational view of a coil made in accordance with one embodiment of this invention, outer portions of the coil being cut away in part for purposes of illustra tion;

FIG. 2 is an end view of this coil;

FIG. 3 is an enlarged, fragmentary sectional view taken diametrally through the axis of this coil;

FIG. 4 is an enlarged, fragmentary part-elevational, part sectional view generally similar to FIG. 3 but illustrating a modified coil made in accordance with the second embodiment of this invention;

FIG. 5 is a fragmentary perspective view of a multilayered blank, which is used in making a still further embodiment of this invention;

FIG. 6 is a perspective view of a coil made from the blank illustrated in FIG. 5;

FIG. 7 illustrates diagrammatically the manner in which the coil of FIGS. 1 to 3 is utilized for purposes of welding;

FIG. 8 is a wiring diagram illustrating the use of this coil as ballast for the ignition circuit of a fluorescent lamp;

- FIGS. 9 and I0 illustrate the use and function of the coil of FIG. 4 as a low-pass filter;

FIG. 11 is a wiring diagram illustrating the use of the coil of FIG. 4 as a delay element in an electric circuit;

FIG. I2 is a wiring diagram illustrating the use of the coil of FIG. 6 as an oscillator;

FIG. I3 is side elevational view of a modified coil made in accordance with still another embodiment of this invention, parts thereof broken away and shown in section; and

FIG. 14 is an end view of the coil shown in FIG. 13.

Referring now to the drawings by numerals of reference, and first to the embodiment shown in FIGS. I to 3, 20 denotes a multilayer coil mounted on a core or spool 22, which has an annular shank 23, integral end flanges 24 and 25, and an axial bore 26 for accommodating a soft iron rod, if desired. Core 25 is made from an inexpensive, electrically nonconductive material such as wood or plastic.

Secured around the outside of the hub 23 in contact therewith is a thin layer or sleeve 28 of electrically conductive foil, which, for example, is approximately one-half mil thick, and may be made, for instance, from aluminum or brass. Starting adjacent the flange 24 (FIG. 3), a single, insulated wire conductor 30 is wound helically and coaxially around the outside of sleeve 28 toward the opposite spool flange 25 to form a first or inner layer 30-1 of the coil. Layer 30-1 is enclosed in a thin (e.g., one-half mil) layer or sleeve 32 of elastic, nonlinear, dielectric insulating material of the type, for example, sold under the trade name Ger-Pak." Sleeve 32 is enclosed in a second, electrically conductive foil sleeve 34 similar to sleeve 28, and conductor 30 extends helically around the foil sleeve 34 back toward the flange 24 to form a second layer 30-2 of the coil. Another dielectric sleeve 36 surrounds the wire layer 30-2, and another foil sleeve 38 surrounds the insulating sleeve 36. The helically wound conductor 30 continues alternately back toward the flange 25 and then to the flange 24 to complete two remaining layers 30-3 and 30-4 of the coil, with layer 30-3 enclosed in insulation and foil sleeves 40 and 42, respectively, and the layer 30-4 enclosed in an insulating sleeve 44 and a foil sleeve 46, which forms the outermost layer of the coil. Opposite ends of conductor 30 extend externally of coil as at L1 and L2 to form a pair of leads or terminals for connecting coil 20 in an electric circuit.

In use, the coil 20 may be connected by its leads LI and L2, in a welding circuit for instance, for operation as a high-voltage inductance coil, as illustrated in FIG. 7. In this event a DC power supply of, for example, 300 volts, is applied through an ammeter A to a welding electrode 48 carried by a conventional handle 49. The work W, which is to be welded, is connected by the coil lead LI through the coil 20 and lead L2 to the negative terminal of the voltage supply. When a switch S1 in the power supply circuit is closed, an electric arc or spark intense enough to melt the work W is developed between the electrode 48 and the work W. I

FIG. 8 shows the coil 20 used as ballast for the ignition circuit of a conventional fluorescent lamp 50. In this system the electrically insulated housing of a temperature-responsive switch S2 is secured to the outer surface of the coil in contact with the outer layer 46 thereof. Switch S2 has a normally closed contact 51, which is connected at one side to the terminal T1 of a conventional two-pole, single throw switch S3, and at its opposite side through lead LI, coil 20, lead L2 and the lamp 50 to terminal T2 of switch S3. Switch S2 also has a normally open contact 52, which is connected at one side through a resistor R1 to terminal TI, and at its opposite side through lamp 50 to terminal T2.

When the switch S3 is closed, an AC power supply of, for example, 1 15 volts, is applied across the terminals TI and T2, so the current flows through the normally closed switch contact SI, the coil 20 and the conventional starter (not illustrated) in the lamp 50. The lamp is immediately ignited; and at the same time the temperature of the coil 20 increases rapidly, thereby causing temperature-responsive switch S2 to open its contact 51, and to close its contact 52, so that the coil 20 is disconnected from the circuit, and the resistance RI, which may be of approximately 50 ohms, is inserted in the circuit in place of coil 20. As soon as the lamp 50 is illuminated, the switch S3, which, if desired, may be in the form of a conventional, spring-loaded pushbutton-type switch, is moved to its open position.

Referring now to FIG. 4, wherein like numerals are employed to designate elements similar to those employed in the first embodiment, 60 denotes a modified coil in which the annular body portion 23 of its core 22 is enclosed in an inner foil sleeve 61 made of the same material as the foil sleeves employed in coil 20. Also, as in the first embodiment, a single, insulated wire conductor 62 is wound alternately back and forth between opposite ends of the core to form thereon a coil having four, radially spaced, concentric wire layers 62-1, 62-2, 62-3 and 62-4; In this embodiment, however, the innermost coil layer 62-1 is enclosed successively in a foil sleeve 63, an insulating sleeve 64, and a further foil sleeve 65. Similarly the second coil layer 62-2 is enclosed successively in sleeves 66, 67and 68 of foil, insulation and foil, respectively; and the third coil layer 62-3 is enclosed successively in sleeves 69, 70 and 71 of foil, insulation and foil, respectively. The outermost coil layer 62-4 is enclosed in an outer foil sleeve 72. As in the case of the first embodiment, the foil sleeves employed in the coil 60 may be made of, for example, aluminum or brass, and

the insulating sleeves are made of nonlinear elastic, dielectric material.

Each of the wire layers 62-1 through 62-4 of coil 60 is thus enclosed in two separate foil sleeves. Opposite ends L3 and L4 of the wire conductor 62 project out of the associated core, as for example through small openings in the end flanges 24 and 25. Also, as illustrated schematically in FIG. 4, commencing with the innermost foil sleeve 61, alternate foil sleeves 61, 65, 68 and 71 are connected adjacent the spool flange 24 to a common lead L5, which also extends exteriorly of the spool; and adjacent the flange 25, the intervening foil sleeves 63, 66, 69 and 72 are also connected to a common lead L6, which also extends exteriorly of the spool. Coil 60 is thus provided with four external leads, two of which (L3 and L4) can be used to connect the wire conductor 62 in a circuit, and the other two of which (L5 and L6) can be used to connect the foil sleeves of coil 60 in circuit.

FIGS. 9 and 10 illustrate the use of coil 60 as a low pass filter. In this case the output of an oscillator 75 is connected across the leads L3 and L5 of the coil 60; and leads L4 and L6 are connected to opposite ends of a resistor R2. A plot of the signal frequency versus the voltagetransfer ratio between the input and the resistor R2 (FIG. 10) shows that for voltages of relatively low frequencyi.e., up to 10 cycles per second (Hertz) -the voltage transfer ratio remains substantially constant, and slightly below unity. For higher frequencies, however, in the range of, for example, 10 c.p.s., the transfer ratio drops off rapidly.

The same coil 60 exhibits excellent signal delay characteristics, when interposed in a circuit as shown in FIG. 11. In this case the output of a pulse generator 77 is applied through an oscilloscope 78 and a multicontact switch S4 selectively either directly back to the pulse generator 77, or through the coil 60 and then back to the generator 77. With switch S4 disposed in its reference or REF" position (FIG. 11), successive input signals, one of which is illustrated at 79 (solid lines) on the face of scope 78, are returned directly to the generator 77 after being displayed on the scope 78. With switch S4 in its Test position, the lower two contacts of the switch are open, and the upper two are closed, so that the signals are applied through the coil 60 before returning to the pulse generator. The resultant signals, one of which is illustrated at 80 by broken lines in FIG. 11, lag the reference signals. For a 2- microsecond reference pulse, this lag amounts approximately to a -millisecond interval or delay.

Referring now to the embodiment illustrated in FIGS. 5 and 6, 82 denotes a further coil formed from a rolled or spirally wound blank 83. Blank 83 comprises a layer 84 of tape cable comprising a plurality of spaced, parallel wire conductors 84', which-are embedded side by side in a thin layer of flexible, plastic, dielectric material. At opposite ends A and B of the blank, the wires 84 are connected to common wire leads L10 and L1 I, respectively. Layer 84 is disposed on top of superposed layers 86, 87, 88 and 89 of foil, insulation, foil and insu- Coil 82 is produced by rolling end A of the blank 83 around the outside of the dielectric core 90, so that end B is disposed on the outside of the coil 82, where it may be secured in any conventional manner to prevent the blank from unravelling. The resultant coil has six wire leads L10 through L15, which may be used for connecting the coil in a circuit of the type illustrated, for example, in FIG. 12.

The wiring diagram of FIG. 12 illustrates the use of the coil 82 as an oscillator. In this circuit the output of a signal generator 92 is connected through a resistance R3 to the lead L10 of core 82; and the lead L11 is connected to the positive input terminal of an oscilliscope 93. The lead L13 of the core is connected to the negative input of this scope 93; and the lead L12 is connected to the negative input of a second scope 94. Lead L10 is also connected to the positive input terminal of the scope 94, and through a switch S5 to the lead L13. The leads L14 and L15 are left open.

The signal input to the circuit from the generator 92 is illustrated schematically by the signal representation illustrated on the face of the scope 94. When the switch S5 is open, the signal output of the coil 82, as represented at 96 on the face of the scope 93, decays at a constant rate, producing a sawtooth signal. However, when the switch S5 is closed to short out leads L10 and L13, the resultant signal, as illustrated at 97 on the scope 93, oscillates rapidly as it decays. By keying the circuit through the opening and closing of the switch S5, the coil 82 can be used to develop a coded radio signal, for example. For a 2-microsecond pulse input, the coil 82 produces, when the switch S5 is closed, a decay pulse of approximately 50 microseconds.

Referring now to FIGS. 13 and 14, wherein like numerals are again employed to denote elements similar to those employed in the previously described embodiments, 100 denotes a coil in which each of the spool flanges 24 and 25 has therein a plurality of arcuate openings 102, which are arranged in a plurality of concentric, radially spaced circular paths so that the openings in adjacent paths are angularly offset from one another about the spool axis.

Mounted adjacent opposite ends thereof in registering openings 102 in the flanges 24 and 25 are a plurality of elongate, parallel, magnetizable metal strips or shims 103, opposite ends of which project for equal distances beyond opposite ends, respectively, of the spool 22. In the embodiment illustrated, four radially spaced rows of shims, designated at A, B, C and D, respectively, are provided. The shims of group A, which is the radially innermost of the four groups, are radially spaced slightly from the outer peripheral surface of the shank of spool 22; and the shims 103 of one group are angularly offset about the axis of the spool relative to the shims in the next adjacent group.

Commencing adjacent flange 24, a single, insulated wire conductor 105 is repeatedly wound helically coaxially around i the outside of the spool 22 alternately to the opposite spool flange 25, and then back again to flange 24, to form on the spool five, radially spaced wire layers. Each successive layer is enclosed in a layer 106 of brass or aluminum foil of the type previously described; and each of these, in turn, is enclosed in layer 107 of relatively heavy gauge dielectric material such as, for example, the above-noted Ger-Pak."

The innermost group A of shims 103 is positioned around the outside of the innermost dielectric layer 107, and the conductor 105 is wound in a helical path around the outside of these shims, and the intervening portions of the adjacent dielectric layer 107, from spool flange 25 toward opposite spool flange 24. This layer of the coil is then enclosed within the nest cylindrical layer 106 of foil, which in turn is enclosed in the second layer 107 of dielectric material. The shims of group B are then enclosed in the next layer or helical winding of conductor 105; and in similar manner, the next foil and dielectric layers 106 and 107 are interposed between the last coil winding and the shims of group C. The shims of group C are enclosed successively within the next coil winding, the next foil layer 106 and the next dielectric layer 107; and the shims of group D are enclosed within the fifth coil winding, the last foil layer H06, and the outermost or final layer 107 of dielectric material. Opposite ends of the foil layers or sleeves 106 may then be connected to wire leads L23 and L24 as shown in FIG. 13.

As shown more clearly in FIG. 13, each of the several openings 102 in the end flanges 24 and 25 has a cross-sectional area that is slightly larger than the cross-sectional area of the shim 103 extending therethrough. Projecting from the inside faces of the flanges 24 and 25 adjacent the radially innermost edges of the openings 102 are a plurality of spaced fulcrums 109, each of which engages the radially inner side of a shim 103. This permits the ends of the shims 103 to vibrate radially of the core, when coil 100 is used in a generator of the type disclosed in my above-noted pending application Ser. No. 800,383. For a detailed description of the manner in which coil 100 is employed in a current generator, reference may be made to said application.

From the foregoing it will be apparent that the novel coils disclosed herein incorporate integrated inductance and capacitance characteristics, which can be readily utilized in various circuit applications. Unlike conventional coils of the variety that are designed to eliminate stray capacitance, the herein-disclosed coils positively incorporate capacitance for the purpose of storing electrostatic as well as magnetic energy. Thus by proper interconnection of a coils wire windings and its insulated foil sleeves or cylinders, the effect of the electrostatic energy storage can either be emphasized or deemphasized. while in conventional coils a large inductive reactance and a generally small capacitive reactance and resistance are present, the coils disclosed herein are designed so that they can incorporate comparable size resistance, capacitive reactance and inductive reactance, which then-can be utilized by the wire leads which extend to the exterior of a coil, and which are selectively connected to the wire and foil windings thereof.

Moreover, while a conventional coil acts as a choke, the coils disclosed herein, depending upon the configuration of the respective coil, and the interconnections of the leads to its foil and wire windings, respectively, can be operated selectively as chokes, filters, for storage of impulse energy, as delays, or as absorbers of the impulse energy. Furthermore, by distributing magnetizable metal strips in radially spaced, axially extending relation throughout the coil, the coil is particularly suitable for use in an electric current generator of the type noted herein.

Having thus described my invention, what i claim is:

1. An integrated inductance and capacitance device comprising a core,

an insulated wire conductor wound helically around said core first from one end thereof toward the other and then back to form a plurality of coaxial wire windings around said core,

a pair of metal foil sleeves disposed at opposite sides, respectively, of each of said windings coaxially thereof, whereby each of said windings is enclosed in a separate pair of said foil sleeves,

a separate, sleeve-shaped layer of dielectric material interposed between adjacent foil sleeves electrically to insulate said foil sleeves one from the other,

opposite ends of said wire conductor projecting exteriorly of said sleeves to provide a first pair of leads for connecting said conductor in an electric circuit,

a second pair of wire leads projecting exteriorly of said sleeves, one of said second pair of leads being electrically connected at one end of said core to the adjacent ends of alternate foil sleeves of said device, and

the other of said second pair of leads being electrically connected at the opposite end of said core to adjacent ends of the intervening foil sleeves of said device.

2. An integrated inductance and capacitance device, comprising a core,

a single, insulated wire conductor wound helically around said core from one end thereof toward the other and back repeatedly to form a plurality of coaxial wire windings around said core,

a separate layer of metal foil surrounding, and in contact with the outside of each helical winding,

a layer of thin, flexible, dielectric material surrounding each of said layers of metal foil electrically to insulate each said foil layer from the next wire winding,

said core being a tubular nonmagnetic member, and

a plurality of separate, elongate metal strips mounted on said core between said windings to extend in the direction of the axis of said core and in angularly spaced relation about said axis,

each of said strips projecting at opposite ends thereof beyond opposite ends, respectively, of said core.

3. A device as defined in claim 2, wherein there are at least three radially spaced layers of said windings surrounding said core coaxially thereof,

there are two radially spaced groups of said strips surrounding said core and separating said windings, and

the strips of one of said groups are angularly offset about said axis relative to the strips of the other of said groups.

4. A device as defined in claim 3, including spaced fulcrum means engaged with each of said strips adjacent one end of said core to support the adjacent, projecting ends of said strips for vibration radially of said axis.

5. An integrated inductance and capacitance device, comprising a core,

a plurality of layers of insulated wire windings surrounding said core, and

a plurality of layers of thin, flexible material separating adjacent wire layers,

one of said separating layers comprising a sheet of metal foil, and

another of said separating layers comprising a sheet of dielectric material,

said windings comprising a plurality of spaced wire conductors embedded in a layer of flexible, dielectric material,

said dielectric material being wound spirally around said core,

means connecting the inner ends of said conductors to a first wire lead, which extends exteriorly of said spirally wound layers, and

means connecting the outer ends of said conductors to a second wire lead, which extends exteriorly of said layers.

6. An integral inductance and capacitance device, comprising a plurality of coaxial layers of insulated wire windings,

a plurality of layers of thin material separating each two successive windings,

one of said separating layers being a sheet of electrically conductive metal foil,

another of said separating layers being a sheet of elastic,

dielectric insulating material, and

a plurality of separate, angularly spaced thin iron strips extending axially between adjacent layers of said windings and projecting at opposite ends thereof axially beyond said windings, and spaced fulcrum means adjacent one end of said core on which said strips seat. 

1. An integrated inductance and capacitance device comprising a core, an insulated wire conductor wound helically around said core first from one end thereof toward the other and then back to form a plurality of coaxial wire windings around said core, a pair of metal foil sleeves disposed at opposite sides, respectively, of each of said windings coaxially thereof, whereby each of said windings is enclosed in a separate pair of said foil sleeves, a separate, sleeve-shaped layer of dielectric material interposed between adjacent foil sleeves electrically to insulate said foil sleeves one from the other, opposite ends of said wire conductor projecting exteriorly of said sleeves to provide a first pair of leads for connecting said conductor in an electric circuit, a second pair of wire leads projecting exteriorly of said sleeves, one of said second pair of leads being electrically connected at one end of said core to the adjacent ends of alternate foil sleeves of said device, and the other of said second pair of leads being electrically connected at the opposite end of said core to adjacent ends of the intervening foil sleeves of said device.
 2. An integrated inductance and capacitance device, comprising a core, a single, insulated wire conductor wound helically around said core from one end thereof toward the other and back repeatedly to form a plurality of coaxial wire windings around said core, a separate layer of metal foil surrounding, and in contact with the outside of each helical winding, a layer of thin, flexible, dielectric material surrounding each of said layers of metal foil electrically to insulate each said foil layer from the next wire winding, said core being a tubular nonmagnetic member, and a plurality of separate, elongate metal strips mounted on said core between said windings to extend in the direction of the axis of said core and in angularly spaced relation about said axis, each of said strips projecting at opposite ends thereof beyond opposite ends, respectively, of said core.
 3. A device as defined in claim 2, wherein there are at least three radially spaced layers of said windings surrounding said core coaxially thereof, there are two radially sPaced groups of said strips surrounding said core and separating said windings, and the strips of one of said groups are angularly offset about said axis relative to the strips of the other of said groups.
 4. A device as defined in claim 3, including spaced fulcrum means engaged with each of said strips adjacent one end of said core to support the adjacent, projecting ends of said strips for vibration radially of said axis.
 5. An integrated inductance and capacitance device, comprising a core, a plurality of layers of insulated wire windings surrounding said core, and a plurality of layers of thin, flexible material separating adjacent wire layers, one of said separating layers comprising a sheet of metal foil, and another of said separating layers comprising a sheet of dielectric material, said windings comprising a plurality of spaced wire conductors embedded in a layer of flexible, dielectric material, said dielectric material being wound spirally around said core, means connecting the inner ends of said conductors to a first wire lead, which extends exteriorly of said spirally wound layers, and means connecting the outer ends of said conductors to a second wire lead, which extends exteriorly of said layers.
 6. An integral inductance and capacitance device, comprising a plurality of coaxial layers of insulated wire windings, a plurality of layers of thin material separating each two successive windings, one of said separating layers being a sheet of electrically conductive metal foil, another of said separating layers being a sheet of elastic, dielectric insulating material, and a plurality of separate, angularly spaced thin iron strips extending axially between adjacent layers of said windings and projecting at opposite ends thereof axially beyond said windings, and spaced fulcrum means adjacent one end of said core on which said strips seat. 